Charge storage sheet with tapered apertures



Aug. 17, 196 R. K. ORTHUBER ETAL 3,291,639

CHARGE STORAGE SHEET WITH TAPERED APERTURES Filed March so, 1955 +IV. ERASE IM. I;

III III! WRITE Q-eo F/GB 1NVENTOR RICHARD K. ORTHUBER CHARLES V. STANLEY a 1am V ATTORNEY United States Patent 3,281,639 CHARGE STQRAGE SHEET WITH TAPERED APERTURES Richard K. Orthuher and Charles V. Stanley, Fort Wayne,

lincL, assignors to International Telephone and Telegraph Corporation Filed him. 30, 1955, Ser. No. 497,346 4- Claiins. (Cl. 313

T he present invention relates to a charge storage device and more particularly to a storage screen for an electron discharge tube.

image storage tubes of the type capable of storing a radiation image for long periods of-time, during which periods the image may be reproduced on a luminescent screen, are limited in sensitivity by two factors: (1) the sensitivity of the photocathode and (2) the capacity of the storage screen. Storage image tubes are now wellltnown in the art and one type of image storage tube is covered in Farnsworth application Serial No. 413,748, filed March 3, 1954, now Patent No. 2,992,358. This prior application may be referred to for certain details of construction and operation of the invention disclosed hereafter, which details are now common knowledge to a person skilled in the art. Other typical storage tubes are disclosed and claimed by Farnsworth Patents 2,228,388 and 2,251,124.

The capacitance of the storage screen of these earlier tubes was approximately one thousand (1,000) micromicror'arads per square centimeter.

Prior art storage screens consisted of a fine mesh metallic screen onto which a thin layer of a dielectric, such as silicon oxide, was evaporated. Screens of this type have a capacity of approximately one thousand (1,000) micromicrofarads per centimeter. One effort to decrease the capacity of this storage screen was to prepare the screen as above and thereafter remove the metallic mesh. The resulting product was a fine mesh silicon oxide screen, the capacity of which was adjustable to very low values by spacing proper distances from the metal electrodes of tube. While this screen possesses the desired low capacity, its rigidity is not entirely satisfactory for practical use in a tube.

accordance with this invention, it was found that a reduction in screen capacity is possible by increasing the However, increasing the thick- 'nrss Oi rne insulator. ness alone does not solve the problem as is explained in more detail hereinafter, since it is necessary to utilize tapered screen apertures in order to obtain optimum storage time.

it is, therefore, an object of this invention to provide a storage screen for an image tube having low capacity.

another object of this invention to provide a relatl 'ck storage screen capable of storing a charge 11 e for a relatively long period of time.

ther features and objects of this invention and the manner of attaining them will become apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a longitudinal section of an embodiment of this invention;

G. 2 is a fragmentary elevation of the storage screen 16. 1; and

able power supply it as shown.

ii dlfidd Patented Aug. 17, 1965 "ice FIG. 3 is a sectional view taken substantially along line 3-3 of FIG. 2.

Referring to the drawings, an evacuated tubular glass envelope 1 is provided at its opposite respective ends with a photoelectric cathode 2 and a phosphor screen anode 3. The cathode 2 is of conventional design and may be composed of any suitable photoelectric material such as cesium-oxide-silver. The phosphor screen 3 is conventional and may be made of the same material as that used in ordinary television picture tubes. Interiorly of the tube are mounted three accelerating anode rings or elements 4, and adjacent thereto is a coarse mesh metallic collector screen 5. The storage screen 6 is fixedly mounted between the collector 5 and the phosphor screen 3.

The storage screen 6 is composed of a relatively thick layer 7 of insulating material having a secondary emission ratio greater than unity, and a conductive coating 3 on one side thereof, which coating may be composed of evaporated metal such as aluminum. A transparent conductive coating of Nesa or stannous chloride may be used instead. in order to achieve a high secondary emission ratio the entire screen on its insulated surfaces may be coated with a highly secondary emissive insulator film such as magnesium oxide.

A plurality of tiny tapered apertures 9 are provided in the body 7 which open toward the phosphor screen 3, the coating 8 being on the side of the screen having the larger diameter apertured portions. Suitable dimensions for the screen are 2.7 mils thickness with the number and spacing of the apertures 9 corresponding to a three hundred (398) mesh size screen. The degree of taper for the apertures 9 is critical as will be explained in more detail hereinafter and should range in the illustrated embodiments from between ten degrees to twenty degrees (10 to 20). However, as will become apparent from the following description, other factors enter into the determination of optimum taper, whereupon it is possible to utilize tapers of diiierent sizes. However, for the physical screen dimensions just given, a twenty degree (20) taper has been found to give excellent results.

in operation, external connections are made to a suit- The photocathode 2 is normally grounded, through a switch ii, the three accelerating anodes t, have progressively larger potentials ap plied thereto, and the conductive backing it for the storage screen 6 is normally at ground potential. The collector screen 5 is maintained at a potential positive with respect to the storage screen 6 while the phosphor screen 3 has a high positive potential applied thereto. While a simple switch 11 is shown, it will be obvious that any equivalent device may be used instead, such as an electronic switch.

In operation, assuming static conditions under which the storage screen 6 is in readiness for accepting a charge image, a relatively large negative voltage, such as three hundred (300) volts, is applied to the cathode 2 by means of the switch 11. Simultaneously, a potential of approximately minus 1 volt is applied to the backing 3. A radiation image is projected onto the photo-cathode by means of a suitable lens 12 which thereupon emits a pattern of electrons corresponding to the image. This electron image is extended through the tube by means of the accelcrating sleeves 4 to impinge upon the front face 13 of the storage screen 6. Focusing of this electron image on the ing Chemistry, 7 -vol. 46, page 174, January1954, which may be converted" 'to a oeramic material after etching.

- .23 screen 6 is achieved by use of a tubular permanent magnet 14 which surrounds the tube 1. By reason of the relatively great difference of potential between the cathode 2 and the storage screen backing 8, the velocity of the electrons impinging upon the front face 13 is suflicient to produce secondary emission. With the material 7 having a secondary emission ratio greater than unity, a positive charge pattern or image will form on the storage face 13. If the material 7 has a ratio less than unity a negative charge pattern will form.

Next, in order to reproduce the charge image on the screen 3, the photocathode 2 is grounded and the screening backing 8 is also grounded. The photocathode 2 is uniformly'illuminated by means of a suitable lamp which causes the emission of a beam of flood electrons of uniform cross-section. These electrons are directed toward and onto the front surface 13 of screen 6; however, since a relatively high positive potential prevails on the phosphor screen 3, the electric field therefrom draws the electrons through the screen apertures 9. These electrons then impinge upon the screen 3 to produce luminescence. The electrons as'they pass through the apertures 9 are modulated by the elemental charges of the stored image surrounding the respective apertures9, whereupon the flood electrons finally impinging the phosphor are density modulated into a replica of the stored image and serve to produce a visible image on screen 3.

The charge image on the storage screen 6 may be erased simply by applying a potential of about one (1) volt positive, for example, to the metal backing 8 and flooding the cathode 2 with light from the lamp 15 with the cathode 2 grounded. This serves to neutralize or erase the positive charge image on the storage screen 6.

While specific examples of voltages have been given, it will be understood by a person skilled in the art that the exact voltages used will depend upon desired tube performance and physical dimensions of the tube elements. Also, while a photoelectric cathode 2 has been disclosed for the purpose of producing the charge image on the 'screen 6 as well as for producing the flood electrons, it is of course apparent that an electron gun and a separate flood cathode'may be used instead of as described in Farmworth Patent No. 2,228,388. Since the mode of operation of a storage tube is well understood by persons skilled in the art,'no further elaboration is necessary herein.

The insulating material 7 of the storage screen may be quartz, ceramic, or a similar substance. In the drawings, the material 7 is in sheet-like form with parallel front and back faces. A more convenient usable material than quartz is a photosensitive glass of the type described in two different publications found in Industrial &' Engineervol. 45, page 15, January 1953, and in With the'storage screen made as described, an image may be stored and reproduced over relatively long periods of time. The screen is found to have an'optimurn value of lowcapacity, thereby increasing 'tube sensitivity. Furthermore, flooding of the screen 6 in order to obtain a reproduction of the charge image is found to have either no or only negligible erasure or neutralizing'eifect on the charge image. This last-mentioned feature of operation isbest explained by considering the example of the aper tures 9 being cylindrical instead of tapered as shown. With a charge image. impressed on the front face of the storage screen, flood electrons passing through the indi vidual apertures will impinge the aperture walls, thereby producing a charge which offsets or neutralizes the charge image on-the front face 13. In actual experiments, it

was found that relatively thick storage screens provided with cylindrical apertures or parallel-wall apertures could In order to prevent electron impingement of the apercathode 2 as shown in detail in FIG. 3, whereby elec trons following the paths as indicated by the reference numeral 16 will not be able to strike the aperture walls. Thus, it is now apparent that While a taper of 20 degrees may be suitable for one tube design, other tapers may be used depending upon electron velocity, the thickness of the storage screen, and the proximity of the phosphor screen 3 to the storage screen 6. Also, the magnitude of the voltage applied to the phosphor screen 3 is a determining factor. In any event, the taper must be adequate to prevent flood electrons from impinging or being collected by the aperture Walls.

While the screen 6 has been specified as having a secondary emission ratio greater than unity, this is merely exemplary only since the emission ratio may be less than unity for reproducing a negative image. The scope of this invention isintended to include screens having either greater than or less than unity secondary emission ratio. Tubes made according to the foregoing description are found to possess materially improved sensitivity, and to be capable of storing images over relatively long periods ai time. j

While it will be understood that the specifications for :the tube of theinvention may vary for any particular applicatiom the following specifications for a tube are included by way of example only as suitable for converting infra-red images into visible images.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitationto the scope of our invention.

What is claimed is:

1. For use in an electron discharge image tube, a storage screen comprising a sheet-like member of insulating material having flat parallel faces, a plurality of apertures through said member, said apertures being tapered inwardly from one face of said member to the'other, the

degree of aperture taper being sufiicient-toprevent flood electrons from impinging upon the aperture walls, and

a coating of conductive material on said one face.

2. An electron discharge tube havingseparated cathode and anode elements, a storage screen interposed in the path of electron flow between said elements, said screen comprising a sheet-like member of insulating material having fiat parallel faces, a plurality of tapered apertures not storefimages for more than a few seconds, when impinged with flood electrons.

in said member, the narrow ends of said apertures facing the cathode element and the Wide ends of said apertures facing the anode, and a coating of conductive material on the anode face of 'said member.

i 3. An electron discharge tube having separated cathode and anode elements, a storage screen interposed in the path of electron flow between said. elements, said storage screen comprising a solid body of insulating material having flat parallel faces with a plurality of apertures formed therein for receiving therethrough a stream of electrons from said cathode element, said apertures being tapered with their narrow ends facing said cathode ele ment thereby to prevent electrons from impinging upon I the aperture walls.

a 6 the path of electron flow between said elements, said screen References Qited by the Examiner comprising a solid sheet-likebody of insulating material UNITED STATES PATENTS having flat parallel faces With a plurality of apertures formed therein, said apertures being tapered with their 2,513,743 7/50 Ralchman 313-68 narrow ends facing said cathode element thereby to pre- 5 2,547,638 4/51 Gardner 313-58 vent electrons from impinging upon the aperture Walls, 2,824,249 2/58 Hansen 313-68 and a screen formed of conductive material abutting the anode face of said body with its openings respectively in GEORGE WESTBY Primary Examiner registry with said apertures. NORMAN H. EVANS, Examiner. 

1. FOR USE IN AN ELECTRON DISCHARGE IMAGE TUBE, A STORAGE SCREEN COMPRISING A SHEET-LIKE MEMBER OF INSULATING MATERIAL HAVING FLAT PARALLEL FACES, A PLURALITY OF APERTURES THROUGH SAID MEMBER, SAID APERTURES BEING TAPERED INWARDLY FROM ONE FACE OF SAID MEMBER TO THE OTHER, THE DEGREE OF APERTURE TAPER BEING SUFFICIENT TO PREVENT FLOOD ELECTRONS FROM IMPINGING UPON THE APERTURE WALLS, AND A COATING OF CONDUCTIVE MATERIAL ON SAID ONE FACE. 